In most cases, steel plates are manufactured with the expectation that they will be welded. Generally speaking, the portion affected by welding heat will change as described below, although this will vary depending on the welding method.
A .STRUCTURE of the Welding Heat-affected Zone
Diagram of Fe-C equilibrium state and its relationship with the welding thermal cycle
Table 1: Structure of the weld heat-affected zone of steel
B. Hardening of HAZ
As the weld metal (WM) of non-heat treated steel plates has a kind of cast structure, the heat affected zone (HAZ) usually increases more in hardness due to the hardening effect than the base metal (BM). The harder the HAZ, the lower the ductility, leading to probable cold cracking in welding or use. Therefore, steel grades and welding conditions must be determined carefully so that the maximum hardness of HAZ may be as low as possible. The carbon equivalent is widely used to estimate the maximum
hardness. The relationship between the maximum hardness and the carbon equivalent is shown in Fig. below.
Relationship between the maximum hardness and the carbon equivalent
Maximum hardness
（Hmax）＝a・Ceq＋b
（a and b are constants that depend on joint conditions.）
Carbon equivalent
In contrast, as the HAZ of heat-treated steel plate is heated at a high temperature above tempering temperature, softening phenomenon occurs.（ refer below Fig. ）
Hardness of weld zone
Therefore, it is necessary to conduct welding work with weld heat input as small as possible.
Source: nssmc.com
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